The result of the MAP order was the NA-73X project (from March 1940). The design followed the best conventional practice of the era, but included two new features. One was a new NACA-designed laminar flow wing, which was associated with very low drag at high speeds. Another was the use of a new radiator design (one Curtiss had been unable to make work) that used the heated air exiting the radiator as a form of jet thrust in what is referred to as the "Meredith Effect". Because North American lacked a suitable wind tunnel, it used the GALCIT 10-foot wind tunnel at Cal Tech. This led to some controversy over whether the Mustang's cooling system aerodynamics were developed by North American's engineer Edgar Schmued or by Curtiss, although historians and researchers dismiss the allegation of stolen technology; such claims are likely moot in any event, as North American had purchased Curtiss’ complete set of P-40 and XP-46 wind tunnel data and flight test reports for US$56,000.
While the United States Army Air Corps could block any sales it considered detrimental or not in the interest of the United States, the NA-73 represented a special case. In order to ensure deliveries were uninterrupted, an arrangement was eventually reached where the RAF would get its aircraft in exchange for NAA providing two free examples to the USAAC for evaluation.
The prototype NA-73X was rolled out just 117 days after the order was placed, and first flew on 26 October 1940, just 178 days after the order had been placed — an incredibly short gestation period. In general, the prototype handled well and the internal arrangement allowed for an impressive fuel load. It was armed with four .30 caliber Browning (7.62 mm) and two .50 M2 Browning (12.7 mm) machineguns in the wings and two .50 M2s in the chin.
